1. Field of the Invention
The present invention relates in general to light emitting diodes, and more specifically, to light emitting diodes using a material of ZnSSe as a transparent layer.
2. Description of Prior Art
There are three prior art LEDs of AlGaInP system in relevance to the present invention. In FIG. 1 there is shown a conventional LED, which is disclosed in Prog. Crystal Growth and Charact', Vol. 19, pp. 97-105 by J.P. Andre et al. The LED of FIG. 1 is fabricated with a back electrode 110, a substrate of n-type GaAs 120, a double heterostructure of AlGaInP 130, which includes a layer of n-type AlGaInP 131, a layer of undoped AlGaInP 132, and a layer of p-type AlGaInP 133, and a second electrode 140. The undoped AlGaInP layer 132 is technically referred to as an active layer, and the two neighboring n-type AlGaInP layer 131 and p-type AlGaInP layer 132 are referred to as confining layers.
For efficient operation of the LED, current injected by the front electrode 140 should be spread evenly to the lateral direction so that current will cross the p-n junction of the double heterostructure of AlGaInP 130 uniformly to thereby generate light uniformly across the same. Since the p-type AlGaInP layer 133 is grown by means of the metal-organic vapor phase epitaxy (MOVPE) process, it is very difficult to dope with acceptors with a concentration of higher than 1.times.10.sup.18 cm.sup.-3. Moreover, it is a material characteristic that hole mobility is low in p-type AlGaInP semiconductor. Due to these two factors, the electrical resistivity of the p-type AlGaInP layer 132 is comparatively high so that lateral current flow from the front electrode 140 is severely restricted. As a result, current injected by the front electrode 140 tends to flow only through the center portions of the double heterostructure of AlGaInP 130 and the n-type GaAs substrate 120 to the back electrode 110. This is often referred to as a current crowding problem.
One technique to solve the current crowding problem is disclosed by Fletcher et al in a U.S. Pat. No. 5,008,718. The proposed LED structure is shown in FIG. 2 (in this figure, layers that are not changed in view of the structure of FIG. 1 are labelled with the same reference numerals), in which a window layer 200 is grown upon the p-type AlGaInP layer 133. The window layer 200 should be selected from materials that have a low electrical resistivity so that current could be spread out laterally, and a bandgap higher than that of the AlGaInP layers so that the window layer is transparent to light emitted from the active layer of AlGaInP.
In an LED for generating light in the spectrum from red to orange, a AlGaAs material is selected to form the window layer 200. The AlGaAs material has an advantage that it has a lattice constant in match with that of the underlying GaAs substrate 120. While in an LED for generating light in the spectrum from yellow to green, a GaAsP or a GaP material is used to form the window layer 200. It is drawback of using the GaAsP or the GaP material that their lattice constants are not in match with that of the AlGaInP layers 130 and the GaAs substrate 120. The lattice mismatch causes a high dislocation density so that optical performance is still not satisfactory.
FIG. 3 shows a third prior art LED disclosed in Photonics Spectra, December 1991, pp. 64-66, by H. Kaplan. The LED of FIG. 3, in addition to the structure of FIG. 1, is fabricated with a reflector layer 310, a current-blocking layer 300, a current spreading layer 330. The current spreading layer 330 has a very low electrical electricity and the current blocking layer 300 is arranged at a position where it is in alignment with the front electrode 140. The current injected from the front electrode 140 thus is spread out laterally by the current blocking layer 300. Moreover, the reflector layer 310 can be used to prevent the light emitted by the active layers from being absorbed by the GaAs substrate.
It is a drawback of the LED of FIG. 3 that the fabricating process, in which the MOVPE procedure needs to be performed twice, is complex. Moreover, the p-type AlGaInP layer 133 is easily oxidized since it contains a large proportion of aluminum.